Recent advances in mobile communication have enabled the devices employed therein to become smaller and more sophisticated. The surface acoustic wave filter (hereinafter referred to simply as a SAW filter) is widely used as a filter in mobile communication apparatuses. SAW filters used in an RF (radio frequency) stage comprise two types: a longitudinally coupled type and a ladder type. The ladder SAW filter has lower loss than a vertical mode SAW filter. The ladder model is formed of a plurality of SAW resonators coupled to each other in a ladder form. Thus, the sophistication of the SAW filter depends on the sophistication of each one of the SAW resonators.
A conventional SAW filter is described hereinafter with reference to FIG. 15. FIG. 15 shows a plan view of a conventional SAW resonator 1500. The SAW resonator 1500 includes an interdigital transducer (IDT) electrode 1504 formed on a piezoelectric substrate 1502 and reflector electrodes 1514 disposed on both sides of IDT electrode 1504.
The IDT electrode 1504 includes a first bus bar electrode 1506; first finger-electrodes 1508 coupled to the first bus bar electrode 1506; a second bus bar electrode 1510; and second finger-electrodes 1512 coupled to the second bus bar electrode 1510. First finger-electrodes 1508 and second finger-electrodes 1512 overlap each other, and both first finger-electrodes 1508 and second finger-electrodes 1512 are arranged such that their respective pitches agree with wavelength λ of the surface acoustic wave (SAW).
Each one of reflector electrodes 1514, disposed on both sides of IDT electrode 1504, is formed of a plurality of strip-line electrodes 1516 electrically shorted to one another by a common reflector electrode 1518. First bus-bar electrode 1506 is coupled to a first terminal 1520, and second bus-bar electrode 1510 is coupled to a second terminal 1522. Thus, SAW resonator 1500 is formed of a pair of terminals.
Use of the conventional SAW resonator 1500 in a SAW filter or an antenna duplexer does not provide desirable filter characteristics or high power durability. Therefore, the conventional SAW resonator has undergone a variety of improvements to its structure.
FIG. 16 shows a plan view illustrating the conventional SAW resonators 1500 coupled in series, thereby forming a SAW resonator 1530. This formation is used for improving the high power durability. SAW resonator 1530 is formed of two resonating units 1532 and 1534 coupled by a connecting electrode 1536 on a piezoelectric substrate 1502. Each one of resonating units 1532 and 1534 has the same structure as SAW resonator 1500 shown in FIG. 15, and thus the description thereof is omitted. A first terminal 1520 and a second terminal 1522 are respectively coupled to the bus-bar electrodes of resonating units 1532 and 1534.
If the capacitance of SAW resonator 1530 is designed to be equal to the capacitance of SAW resonator 1500 shown in FIG. 15, the two resonators will have approximately equal characteristics. In this case, since the SAW resonator 1530 is formed of two resonating units 1532 and 1534 coupled in series, each of their capacitances is designed to be twice as much as the capacitance of SAW resonator 1500. For this purpose, the number of pairs of IDT electrodes forming resonating units 1532 and 1534 is doubled, namely, the number of finger-electrodes of the IDT electrodes is doubled.
FIG. 17 shows a SAW resonator 1540 that employs a common bus-bar electrode 1538 shared by two resonating units 1532 and 1534 in SAW resonator 1530 shown in FIG. 16, so that connecting electrode 1536, as shown in FIG. 16, can be eliminated.
Further, Japanese Patent Application Non-Examined Publication No. H07-74584 discloses a SAW filter which improves the high power durability. The SAW filter includes at least two SAW resonators, wherein each SAW resonator includes a comb electrode for exciting and detecting a SAW and a reflector electrode for reflecting the SAW excited by the comb electrode, where the comb electrode of at least one of the SAW resonators has a split-electrode structure.
Japanese Patent Application Non-Examined Publication No. H08-298433 discloses another SAW filter which improves the high power durability. This SAW filter also employs a plurality of SAW resonators.
A transversal SAW filter, which is not designed to improve the high power durability, assigns weights to an input IDT electrode and an output IDT electrode in order to obtain desirable frequency characteristics. FIG. 18 shows a basic structure of a transversal SAW filter 1542. Transveral SAW filter 1542 is formed of an input IDT electrode 1546 and an output IDT electrode 1548, both placed on a piezoelectric substrate 1544. An input signal Vs is supplied to input IDT electrode 1546 and is transformed into a SAW for propagating to output IDT electrode 1548 on the surface of substrate 1544. The SAW arrives at output IDT electrode 1548, where the SAW is transformed into an electric signal, and then taken out as an output signal across a load RL.
FIG. 19 shows an example of assigning weights to electrodes for changing an exciting strength. This example illustrates only a part of input IDT electrode 1546, which is formed of a first pattern section 1550, second pattern sections 1552 and third pattern sections 1554. In the first pattern section 1550, respective finger-electrodes overlap with each other. In each one of second pattern sections 1552, respective finger-electrodes do not overlap with each other, but one dogleg electrode is placed between the finger-electrodes such that the dogleg is adjacent to the respective finger-electrodes. In each one of third pattern sections 1554, the finger-electrodes also do not overlap with each other, but two dogleg electrodes are placed between the finger-electrodes adjacent to one another. As FIG. 19 shows, the three patterns have different lengths of finger-electrodes. The patterns are optimally arranged along the SAW propagating direction, thereby obtaining desirable frequency characteristics.
FIG. 20 shows an example of assigning weights to electrodes using capacitive coupling. This example shows only one input IDT electrode 1556. In this case, finger-electrodes 1558 and 1560 of IDT electrode 1556 do not overlap with each other but, instead, are placed in a face-to-face manner. A coupling electrode 1562 placed between finger-electrodes 1558 and 1560 forms a capacitive coupling section 1564 and a voltage weighted section 1566 in IDT electrode 1556. The length of coupling electrode 1562 in section 1564 is adjusted to have a length different from other coupling electrodes 5622, so that capacitance C1 belongs to a coupling electrode 5621 and capacitance C2 belongs to coupling electrode 5622. On the other hand, each one of the capacitances in voltage weighted section 1566 is “Ce”.
The foregoing weighting method using capacitive coupling is employed in the transversal SAW filter. In other words, a size of capacitive coupling is changed along the SAW propagating direction, thereby assigning weights. Then the weighting is optimized in order to maintain desirable frequency characteristics in the transversal SAW filter. These techniques are disclosed on pages 195 and 208 of the surface-acoustic-wave handbook edited by the 150th subcommittee of the Science Advancement Council of Japan.
However, since SAW resonator 1530 shown in FIG. 16 includes connecting electrode 1536 and a bus-bar electrode for coupling resonating units 1532 and 1534 in series, it is difficult to downsize the SAW resonator. SAW resonator 1540 shown in FIG. 17 eliminates a coupling electrode, so that it can be downsized accordingly. However, the width of common bus-bar electrode 1538 shared by resonating units 1532 and 1534 is narrowed, and the resistive component due to this narrower width produces negative influences on the characteristic of the SAW resonator 1540, e.g., increasing loss. A greater number of pairs of IDT electrodes, in particular, elongates common bus-bar electrode 1538, and the negative influence becomes more conspicuous.